Field of the Invention
The present invention relates to a switched power converter, and to power converter units to form such converter. In particular the invention relates to a power converter to be used for driving gradient coils in an MRI system.
Description of Related Art
Gradient coils in an MRI system require a high voltage and high current that must be controlled. This voltage is typically in the order of 2000V and above. The required peak current is in the order of 600 A and above.
To generate this high output voltage and high output current a so called stacked H-bridge multi-level power converter is typically used. Such converter may comprise a so called single H-bridge with electronically controlled switches. The bridge is connected to an isolated voltage source, which can be anything that behaves as a voltage source, for example an isolated DC/DC converter or a so called flying capacitor. The switches are controlled with a pulse width modulation scheme suitable for a H-bridge (for example unipolar or bipolar PWM). A stacked H-bridge consists of multiple H-bridges placed in series. The output connection terminals of the converter connect to the load. Such converter is known from the German Patent DE19812069, the Japanese Patent JP2006000645 or the US Patent US2011187369.
There is however a problem with the stacked H-bridges, due to the switching of the H-bridges there is a high dV/dt between each of the H-bridges. This high dV/dt combined with the parasitic capacitances between the separate bridges causes high leakage currents circulating and resonating in the converter. These currents have a negative effect on the converter output signal quality (output voltage and current unwanted high frequency spectral components) and EMC. This problem is addressed in the “Analysis and Suppression of a Common Mode Resonance in the Cascaded H-Bridge Multilevel Inverter (2010), Rixin Lai, Maja Harfman Todorovic and Juan Sabate” wherein a method is proposed to dampen these resonances using strategically placed common mode chokes. Such solution has the disadvantage that it cannot always be realised in practice, due to the requirements set to the control system and/or parasitic effects.
This invention therefore proposes a method to reduce the dV/dt and consequently reducing the circulating leakage currents that takes away the disadvantages of the prior art solutions.